1. Field of the Invention
The present invention relates to a spectral colorimetric apparatus using a diffraction grating for color recognition or colorimetry on a test subject and, more particularly, is suitable for a spectral colorimetric apparatus performing colorimetry on toner or printing media in an image forming apparatus.
The present invention may be applied not only to the image forming apparatus but also to a spectral colorimetric apparatus such as a white balance sensor for a digital camera, and other colorimetric apparatus.
2. Description of the Related Art
In an image forming apparatus for forming a color image through an electrophotographic process, a deviation in color tone may occur because of color mixing of toner. The same problem occurs not only in the electrophotographic type image forming apparatus but also in a general image forming apparatus for forming a color image such as an ink-jet type image forming apparatus. In order to solve the problems as described above, Japanese Patent Application Laid-Open No. H09-160343 proposes a method of measuring a spectral reflection light quantity of a toner image using two different spectral filters and correcting an image signal based on the result of the measurement. Similarly, Japanese Patent Application Laid-Open No. 2004-126278 proposes an image forming apparatus for performing color tint correction on a color image. In the electrophotographic type image forming apparatus according to Japanese Patent Application Laid-Open No. 2004-126278, a sensor for detecting color tint is provided in the downstream of a fixing device located on a fixing and conveying path to detect RGB output values of colors of a mixed color patch image formed on a transferring material (printed image) moving along the conveying path.
In order to determine the color tone at higher precision, it is necessary to increase the number of spectral wavelength bands to at least three, which corresponds to the number of primary colors. When the number of wavelength bands may be further increased, the color tone may be determined at higher precision.
In order to increase the number of wavelength bands, there have been proposed a large number of diffraction spectral devices for performing spectral measurement based on a diffraction phenomenon, for example, in Japanese Patent Application Laid-Open No. 2000-298066, Japanese Patent Application Laid-Open No. H06-058812, Japanese Patent Application Laid-Open No. 2002-206967, and Japanese Patent Application Laid-Open No. H06-094528.
When the diffraction spectral colorimetric device is used, there have been problems that still remain to be solved.
FIG. 12 illustrates a Rowland type diffraction spectral colorimetric device generally used as a conventional diffraction spectral colorimetric device.
A detection light beam which is scattering light from a test subject illuminated by an illumination optical system (not shown) is incident on a detection optical apparatus from an incident window 208. The detection light beam is spectrally separated by a concave surface reflection type diffraction element 204 and then obtained as a spectral intensity distribution by a one-dimensional array detector (one-dimensional array light receiving element) 203. In a case of employing such a structure, a size of the incident window 208 through which the detection light beam enters should be sufficiently small, otherwise there occurs a problem that spectral resolution on the one-dimensional array detector 203 may not be sufficiently high. In order to solve the problem, there is an example in which a stop is provided as the incident window as described in Japanese Patent Application Laid-Open Nos. H06-058812 and 2002-206967.
In the case of the Rowland type diffraction spectral colorimetric device, imaging magnification of the stop on the array detector which is obtained by the diffraction grating is equal-magnification, and hence it is desirable to prepare a stop having a width substantially equal to a width of each of light receiving elements arranged in an arrangement direction on the array detector. In Japanese Patent Application Laid-Open No. H06-058812, the test subject and the stop are made conjugate with each other to image a detection light beam from the test subject on the stop through a mirror and an imaging lens. Such an imaging improves the light use efficiency. According to Japanese Patent Application Laid-Open No. 2002-206967, a detection light beam emerged from an optical fiber is imaged on a stop by an imaging lens.
In the case of the Rowland type diffraction spectral colorimetric device, the relative positions of the stop, the diffraction grating, and the array detector should be aligned with high precision, otherwise the image of the stop blurs on the array detector, and hence sufficient spectral performance cannot be obtained. In view of this, Japanese Patent Application Laid-Open No. H06-094528 discloses an exemplary case of performing focus adjustment by moving a mirror provided between the diffraction grating and the array detector. However, there arises a problem that the number of parts increases in the case of providing a mirror between the diffraction grating and the array detector.
To solve the problem, focus adjustment may be performed by finely adjusting the stop forward and backward. However, in the diffraction spectral colorimetric device in which the detection light beam is condensed on the stop as described in Japanese Patent Application Laid-Open No. 2002-206967 and Japanese Patent Application Laid-Open No. H06-094528, there arises another problem that a light beam passing through the stop changes to vary the quantity of received light reaching the array detector when the stop is moved forward and backward in the optical axis direction. When the quantity of received light varies during the focus adjustment, precise adjustment is made difficult.
Even when the focus adjustment is performed by the system described in Japanese Patent Application Laid-Open No. H06-094528, there arises still another problem that a light beam passing through the stop increases or decreases to vary the quantity of received light reaching the array detector in the case where deviations occur in alignment precision of the stop and in part precision and alignment precision of a light guide element.
In the image forming apparatus for forming the color image through the electrophotographic process as described in Japanese Patent Application Laid-Open No. 2004-126278, it is necessary to read the mixed color patch image on the transferring material (printed image) moving along the conveying path. It is known that the transferring material (printed image) moving along the conveying path vibrates in a direction (upward and downward direction) perpendicular to the conveying direction during the movement. In particular, when a moving speed is fast (printing speed is fast), the vibration becomes significant. In the optical system in which the detection surface and the stop are made conjugate with each other as described in Japanese Patent Application Laid-Open No. H06-058812, when the printed image corresponding to the detection surface vibrates upward and downward (in optical axis direction), the conjugate relationship changes, and hence the focal point on the stop is shifted. That is, there arises further another problem that, a deviation of the focal point of the detection light beam condensed on the stop occurs, and hence a light beam passing through the stop increases or decreases to vary the quantity of received light reaching the array detector, to thereby reduce detection precision.